Although helical gears provide a smoother and quieter transmission of torque between two rotatable shafts, they inherently tend to generate axial forces acting in the direction of the axis about which a gear rotates.
The axially generated forces created when helical gears transmit torque act on the gear unit housing via shaft support bearings. The resulting deformation of the gear unit housing has an adverse effect on the axial clearance or preload of the support bearings. Particularly in the case of bearings of the taper roller type that can have a significant adverse effect on the bearing performance and life, and potentially also an adverse effect on the smoothness and general performance of the helical gears.
It is well known that the axial forces can be avoided by providing a pair of helical gears on a common shaft with one gear of the pair comprising teeth which extend helically in a direction opposite to the teeth of the other gear of the pair. That, however, often is not a satisfactory solution because of additional costs and restriction on other aspects of the gear unit design.
Another technique which may be employed to minimise the effect of axial forces on bearing clearance and preload is to selectively strengthen the housing wall so that it is better able to resist significant defamation. That, however, requires the use of additional material, thereby increasing weight and cost as well as overall dimensions.